In medical practice, hypodermic injections are sometimes administered subcutaneously, while others must be given intravenously, depending upon the particular medication to be administered. In either case, it is essential that the practitioner know with certainty, prior to injection of the medication, whether the hypodermic needle tip is located in a major blood vessel, such as a vein, or in subcutaneous tissue. Use of an aspirating syringe in which a negative pressure can be generated in the syringe affords a means of making such determination. Thus the appearance of blood in the syringe upon generation of the negative pressure would indicate location of the needle tip in a major blood vessel, while the lack of appearance of blood would indicate location of the tip in subcutaneous tissue. Depending upon the type of injection intended, the injection can then either proceed directly, or if appropriate the tip can be withdrawn and relocated.
Aspirating syringes useful for the above stated purpose are generally of two types, that is either manually or automatically actuated. It is conventional in both manual as well as automatic aspirating syringes to use cartridge ampoules of the disposable, pre-loaded type, the lower end of which is closed by a flexible rubber diaphragm, which is piercable by one end of a double-ended needle and secured to the ampoule by a crimped-on overcap, the upper end being closed by a piston slidable within the bore of the cartridge ampoule.
Syringes of the automatic aspirating type are often referred to as self-aspirating syringes. The syringes provided by the present invention are of the latter type and are used in conjunction with disposable cartridge ampoules of the type described above.